1. Field of the Invention
The present invention is broadly concerned with a significantly improved method and apparatus for aseptic processing of pumpable foods, and particularly low acid foods of heterogeneous consistency. More particularly, it is concerned with such a method and apparatus which makes use of a metering device downstream of the heat exchanger and holding tube in order to create a constant flow rate of the product, in conjunction with an initial upstream device for generating a stream of the food product at a substantially constant pressure.
2. Description of the Prior Art
Traditional food preparation and canning processes on a commercial scale involve initially filling a can with the desired food product, followed by sealing of the can and retorting under elevated temperatures and pressures for a sufficient period of time to thoroughly cook the canned product. Such processes have been used for many years, but present a number of significant difficulties. For example, the cost involved in such canning procedures is considerable, not only from the standpoint of energy consumption, but also by virtue of the fact that a wide variety of costly cans must be stocked for filling during the canning season. Further, the rather severe retorting conditions required to cook canned products inevitably tends to degrade the organoleptic properties of the food (e.g., taste, color and particulate identity).
As a consequence, food processors have in the past investigated improved methods of processing foods in order to improve the nutritional and organoleptic properties of the food, while reducing costs. One proposal has been to make use of so-called aseptic processing and packaging. Broadly speaking, such processing involves continuous cooking of a food product prior to packaging in order to sterilize the food, followed by packaging under sterile conditions. Aseptic techniques for processing food were originally developed in the 1940's, but only in recent years have processors begun to widely employ the techniques. The advantages of aseptic processing are many, such as reduced energy and packaging costs, relatively high processing rates, and in some instances the ability to improve organoleptic properties of the food as compared with conventional canning procedures.
Presently, aseptic processing of food is primarily directed to products which are of a homogeneous or fluid nature, such as puddings and juices. Further, many of these products are of the high acid variety (pH below 4.6) where cooking temperatures and holding times are relatively low and there are only negligible health hazards attendant to improper processing.
Aseptic processing of such homogeneous, high acid products generally involves use of a pressurized and sterilized system comprising an initial positive displacement type pump which is used to meter a continuous stream of the food product to a processing section. The latter includes one or more elongated heat exchangers, an intermediate holding tube, and finally a series of cooling heat exchangers. A pneumatic back pressure valve is typically provided downstream of the processing section in order to create a restriction and generate system pressures.
Government regulations covering aseptic processing mandate that sterilization of the product stream is to be determined by the time-temperature conditions in the holding tube. Thus, the time the product is in the initial heat exchanger to bring it up to the sterilization temperature, and the time the product is in any other piping, must be ignored. Hence, in order to establish a safe, commercially viable process, the accuracy of the time-temperature relationship in the holding tube area is of paramount importance.
The government regulations also presently specify that the metering pump shall be located upstream of the holding tube, and shall be operated to maintain the required rate of product flow. Thus, all prior systems meter (i.e., establish substantially constant flow rates of the product regardless of pressure changes) at the upstream end of the processing system ahead of the initial heat exchangers and holding tube.
While systems as described above give acceptable results in the case of high acid, homogeneous products, they are extremely deficient in the case of low acid, heterogeneous products such as soups, stews and chilies. For example, the presence of a back pressure valve cannot be tolerated with non-homogeneous foods, because such valves create a considerable restriction and particulates passing through the valves are inevitably degraded, sometimes to the point of a mush. Thus, other types of equipment have been proposed for use in connection with low acid heterogeneous foods. In one proposal, a rotary positive displacement pump is employed in lieu of the back pressure valve downstream of the processing section, with the downstream pump being operated at a slower speed than the initial metering pump, thereby creating system pressure. Another idea has been to employ a pressurized aseptic surge tank for purposes of pressure generation. Both of these approaches, while theoretically applicable to the aseptic processing of low acid heterogeneous food products, have serious deficiencies. Indeed, these problems are so formidable that there are at present no commercial installations in operation wherein low acid, heterogeneous products are being aseptically processed. This is the case even though there is widespread interest in developing such processes and equipment, for obvious commercial reasons.
A principal problem with the above outlined approaches is the fact that they all involve metering product flow at the upstream end of the apparatus prior to the cooking and holding tube sections. Metering in this context refers to creation of constant product flow rates, regardless of pressure conditions. In any commercial operation though, normal product variations will cause the metering pump to increase or decrease system pressures in order to maintain constant flow rates; thus pressure variations are inevitable. This is significant inasmuch as such pressure fluctuations can lead to localized flashing of steam in the cooking heat exchanger(s). That is to say, it is the conventional practice to operate at system pressures slightly greater (e.g., 10-15 psi) than the pressure required to prevent steam flashing at the cooking temperature being employed. However, if system pressures fluctuate widely, as can occur with upstream metering pumps, conditions can be produced where, in localized regions in the cooking apparatus, pressures are insufficient to prevent flashing. As a result, steam flashes and the product "boils" within the cooking apparatus. This not only disrupts the desirable smooth, continuous flow of product through the apparatus, but moreover can tend to degrade particulate materials. More important, such flashing can disrupt the important time-temperature relationship necessary for proper aseptic cooking, and even lead to an unsterile product.
One possible solution to this problem would be to simply raise system pressures to a point which would clearly compensate for any normal pressure fluctuations encountered in typical operation. However, this proposal creates another difficult problem, because creation of such high pressures using conventional equipment usually results in rather severe processing conditions and hence product degradation. Therefore, the processor is faced with a dilemma when use is made of a constant flow rate metering pumps upstream of the cooking and holding tube apparatus. On the one hand, if system pressures are maintained only slightly above the flash pressure, localized flashing can occur because of pressure fluctuations which are the inevitable by-product of upstream metering. On the other hand, if relatively high pressures are used which would normally preclude any localized flashing this in and of itself can cause product degradation.